Hitherto, in a hydraulic travel drive unit for a vehicle driven by connecting the output torque of a plurality of hydraulic motors through the intermediary of a clutch, the output torque, vehicle speed, and the like are controlled by connecting and disconnecting the clutch when a vehicle speed reaches a predetermined value.
FIG. 6 is a control circuit diagram of a control apparatus for a plurality of hydraulic motors and a clutch of a conventional hydraulic travel drive unit. The control circuit is constituted primarily by a hydraulic pump 50, a first hydraulic motor 51, a second hydraulic motor 52, a clutch 53, and a vehicle speed detection pump 54. The first hydraulic motor 51 and the second hydraulic motor 52 are connected in parallel to the hydraulic pump 50 driven by an engine 15, and are driven by the discharge pressure oil of the hydraulic pump 50. A motor gear 51b is fixedly provided on a first output shaft 51a of the first hydraulic 51, and the motor gear 51b is in mesh with a driving shaft gear 55a of a driving shaft 55 for driving a vehicle. The output torque of the first hydraulic motor 51 is always transmitted to the driving shaft 55 via the motor gear 51b and the driving shaft gear 55a. A wheel 70 is installed on a shaft end of the driving shaft 55.
A clutch 53 is provided on a second output shaft 52a of the second hydraulic motor 52. A second motor gear 53b is fixedly provided on a third output shaft 53a of the clutch 53, and the second motor gear 53b is in mesh with the driving shaft gear 55a of the driving shaft 55 for driving a vehicle. The clutch 53 has a spring 72 therein. The second output shaft 52a and the third output shaft 53a are engaged at a surface S by the spring 72 when no oil pressure is being supplied to an oil chamber 73. When oil pressure is supplied to the oil chamber 73, an oil pressure force overcomes the urging force of the spring 72, causing the surface S to separate thereby to disengage the second output shaft 52a and the third output shaft 53a. 
The output torque of the second hydraulic motor 52 is transmitted to the driving shaft 55 for driving the vehicle through the intermediary of the clutch 53, the second motor gear 53b, and the driving shaft g ar 55a when the clutch 53 is in mesh. A rod distal end of a first cylinder 62 controlled by a first servo valve 61 is attached to one end of a first swash plate 65 of the first hydraulic motor 51. Furthermore, a rod distal end of a second cylinder 64 controlled by a second servo valve 63 is attached to one end of a second swash plate 66 of the second hydraulic motor 52.
The vehicle speed detection pump 54 is connected to the driving shaft 55. The discharge oil of the vehicle speed detection pump 54 is drained into a tank 71 through a throttle 32c. The discharge port of the vehicle speed detection pump 54 is connected to a pressure receiving portion of a tilt rotation fixing control valve 58 and a pressure receiving portion of a clutch switching valve 59. The oil introduced from the tank 71 and discharged from a control pump 56 is set at a constant oil pressure by a relief valve 57, and supplied to port P1 of the tilt rotation fixing control vale 58. Port P2 of a tilt rotation fixing valve 58 is connected to a port P3 of the clutch switching valve 59. A port P4 of the clutch switching valve 59 is connected to the oil chamber 73 of the clutch 53, and a port P5 is connected to the tank 71. The port P2 of the tilt rotation fixing control valve 58 is connected to the pressure receiving portion of a zero tilt rotation fixing valve 60. The drive pressure for driving the second hydraulic motor 52 is supplied to a port P6 of the zero tilt rotation fixing valve 60, and a port P7 thereof is connected to the second servo valve 63.
FIG. 6 illustrates a state of the control circuit when an accelerator pedal is depressed to start acceleration. More specifically, since the vehicle speed is zero, so that the oil pressure output from the vehicle speed detection pump 54 is zero, and both the tilt rotation fixing control valve 58 and the clutch switching valve 59 are set at position “a” by the urging forces of springs 67 and 68. The zero tilt rotation fixing valve 60 is also set at position “a” the oil pressure in the oil chamber 73 is zero, so that the clutch 53 is engaged at surface S by the urging force of the spring 72.
When the acceleration is begun, the first cylinder 62 and the second cylinder 64 are extended and retracted in response to the commands from the first servo valve 61 and the second servo valve 63, causing the first swash plate 65 and the second swash plate 66 of the first hydraulic motor 51 and the second hydraulic motor 52, respectively, to be at their maximum tilts.
FIG. 7 shows a relationship between discharge capacity D (cc/rev) indicating a tilt rotation amount and vehicle speed V. The drive pressure of the second hydraulic motor 52 that decreases as vehicle speed V increases acts on the second servo valve 63 to conduct control so as to reduce the tilt rotation amount of the second swash plate 66 along curve A shown in FIG. 7. As the vehicle speed increases, the discharge volume of the vehicle speed detection pump 54 increases, and the oil pressure on the upstream side from the throttle 32c also increases. When the second swash plate 66 of the second hydraulic motor 52 reaches an approximately zero tilt rotation amount as the vehicle speed increases, that is, when vehicle speed V1 is reached in FIG. 7, the tilt rotation fixing control valve 58 is switched to position “b”. This causes the constant oil pressure output by the control pump 56 through the port P2 of the tilt rotation fixing control valve 58 to be supplied to the pressure receiving portion of the zero tilt rotation fixing valve 60, so that the zero tilt rotation fixing valve 60 overcomes the urging force of the spring 69 and acts at position “b”. Thus, the drive pressure of the second hydraulic motor 52 is supplied to the second servo valve 63 through the port P7 of the zero tilt rotation fixing valve 60. Based on the supplied drive pressure, the second servo valve 63 outputs a command for fixing the position of the second cylinder 64 to the second cylinder 64 thereby to fix the tilt rotation amount of the second swash plate 66. This means that the second swash plate 66 is fixed to the zero tilt rotation amount.
The vehicle speed continues to increase after the second swash plate 66 of the second hydraulic motor 52 is fixed to the zero tilt rotation amount; hence, the oil pressure output by the vehicle speed detection pump 54 continues to increase, and the clutch switching valve 59 acts at position “b” when vehicle speed V2 is reached. Thus, the constant oil pressure output by the control pump 56 is supplied to the oil chamber 73 of the clutch 53 through the ports P1 and P2 of the tilt rotation fixing control valve 58 and the ports P3 and P4 of the clutch switching valve 59. The surface S of the clutch 53 is separated to clear the engagement; therefore, the vehicle is driven only by the first hydraulic motor 51 thereafter.
Meanwhile, in the first hydraulic motor 51, the first swash plate 65 that has been fixed at the maximum tilt rotation amount is released from the maximum tilt rotation amount by an oil pressure signal (not shown) issued when the tilt rotation fixing control valve 58 is switched to position “b”. Then, the tilt rotation amount of the first swash plate 65 decreases according to the drive pressure of the first hydraulic motor 51 that reduces as the vehicle speed increases from vehicle speed V1.
To increase the speed by such a hydraulic travel drive unit, the tilt rotation fixing control valve 58 is first operated at position “b” when the tilt rotation amount of the second swash plate 66 of the second hydraulic motor 52 reaches approximately zero, and the second swash plate 66 of the second hydraulic motor 52 is fixed to the zero tilt rotation amount, then the clutch switching valve 59 is operated at position “b” to disengage the clutch 53. To reduce the speed, the clutch switching valve 59 is first operated at position “a” to engage the clutch 53, then the tilt rotation fixing control vale 58 is operated at position “a” to clear the zero tilt rotation amount of the second swash plate 66 of the second hydraulic motor 52.
However, the conventional hydraulic travel drive unit described above poses the following problem.
The area of the pressure receiving portion of the tilt rotation fixing control valve 58 and the spring 67 are set such that the tilt rotation fixing control valve 58 is actuated at position “a” when the vehicle speed is smaller than vehicle speed V1, and at position “b” when the vehicle speed is the vehicle speed V1 or more. Similarly, the area of the pressure receiving portion of the clutch switching valve 59 and the spring 68 are set such that the clutch switching valve 59 is actuated at position “a” when the vehicle speed is larger than V1 but smaller than V2, and at position “b” when the vehicle speed is V2 or more. However, due to changes in frictional resistance forces of valve spools caused by oil temperature, an increase in leakage due to time-dependent changes of valve spool diameters, or for other reasons, there are cases where a sequence is unsuccessful when the vehicle speed is increased. In the sequence, the second swash plate 66 is first fixed to the zero tilt rotation amount, then the clutch 53 is disengaged to release it. For instance, the clutch 53 may be released before the second swash plate 66 reaches the zero tilt rotation amount. If this happens, there will be a speed change shock when the clutch 53 is released. This causes the second hydraulic motor 52 to race, and all the engine driving force will be undesirably supplied to the second hydraulic motor 52, resulting in no load on the first hydraulic motor 51.
In a normal decelerating operation, the tilt rotation amount of the second swash plate 66 changes from the zero tilt rotation amount after the clutch 53 is engaged. If, however, the tilt rotation amount of the second swash plate 66 starts to change with the clutch 53 still released, then the same problem as that in the accelerating operation arises.